Friday 24 June 2011 13.51 EDT
First published on Friday 24 June 2011 13.51 EDT

Google has been formally notified by the US Federal Trade Commission of a wide-ranging investigation into its dominance of the search advertising market, the company confirmed on Friday.

The widely trailed investigation is the most significant in Google's 12-year history, on a par with the lengthy Department of Justice probe into Microsoft, which led to tighter regulation and from which the technology giant's image has never fully recovered.

Google, which had revenues of $29bn (18.1bn) in 2010, was given notice on Thursday that the FTC will issue "civil investigative demands" – a civil law equivalent to subpoenas – compelling the search giant to share information about its business practices.

One area of concern is whether or not Google's search results rank its own websites and services above those of rivals, which could be seen as anti-competitive. Many critics claim Google automatically places its own websites, such as Google Finance and Google Maps, towards the top of the list in search results.

In a blog published on Friday, Google search engineer Amit Singhal confirmed: "Yesterday, we received formal notification from the US Federal Trade Commission that it has begun a review of our business. We respect the FTC's process and will be working with them."

Google's share price has been falling since Wednesday as news of the inquiry leaked to the financial markets. At the time of publication, it had dropped $19 from Tuesday's closing price of $493, to just under $475.

Google is facing similar probes by the European Commission and the Texas attorney general, but previous FTC inquiries have focused on more peripheral matters such as a proposed partnership with search rival Yahoo! and its acquisition of advertising technology group DoubleClick.

The new FTC inquiry will focus on the heart of Google's search advertising business, which provides most of its revenue. The search engine has a near monopoly position. Despite the best efforts of Yahoo! and Microsoft's Bing, Google accounts for two-thirds of searches in the US, and close to 90% in the UK.

Google has always maintained that access to its rivals is a mouse click away and switching search engines costs nothing. It counters arguments from some quarters that it should be regulated like a utility, and says competition comes from across the internet.

"Using Google is a choice," writes Singhal, "and there are lots of other choices available to you for getting information: other general-interest search engines, specialised search engines, direct navigation websites, mobile applications, social networks and more."